What are you trying to say? So there many users of a legacy OS. So many of them aren't using a good browser. AND? Do you have a point?

Absolutely there is a point.

If you have a website, and you wish to put a video on it, you have to ask yourself "what is the best method for reaching the most users"?

At the moment, the answer is "Flash, with video encoded in H.264". This will cost you however, because you have to pay royalties for sending out video encoded in H.264.

There is at the moment a big push on to move to HTML5, and get rid of the requirement for a Flash plugin for browser clients. If YouTube switch over to HTML5, this will start to happen in a big way. The answer to your question could change ... Flash may soon become no longer the best way to deliver video.

If that happens and users start dropping Flash support in their web browsers, what would be the best way to deliver your video to the most users then?

Given the large number of users still running XP, the answer to this question would then become "HTML5/WebM". This has a bonus advantage to you because you no longer need to pay royalties for your videos sent over the web.

XP will only decrease over time, so it is not a valid reason to push a video format going forward. You have to face the reality that H.264 is already a defacto standard for quality video in just about every arena (web and non-web).